To meet the growing demands of public safety digital radio communications, the federal communication commission (FCC) at the directive of the Congress initiated an inquiry in 1988, to receive recommendations from users and manufacturers to improve the communication systems in existence. Based on the recommendations, in October of 1989 the Association of Public-Safety Communications Officials-International (APCO) Project 25, referred to as P25 and/or APCO-25) came into existence.
Project 25 refers to a suite of standards for digital radio communications for use by federal, state/province and local public safety agencies in North America to enable them to communicate with other agencies and mutual aid response teams in emergencies. In this regard, P25 fills the same role as the European Terrestrial Trunked Radio (TETRA) protocol, although not interoperable with it. The Project 25 defines a radio frequency sub-system (RFSS) as “the smallest portion of infrastructure bounded by the standard APCO interfaces”. The RFSS is expected to provide a set of services (as defined by the APCO standard) across some portion of the system coverage area with an acceptable grade of service. That is, it supports common-air-interface (CAI) and contains all the logic and controlling elements that support call processing and the various APCO open interfaces. The APCO standard for Project 25 allows each equipment vender to design their own solution for an RFSS implementation, as long as the above definition is met.